The majority of outdoor lighting, such as street lamps, are provided with individual controllers which are designed to turn the lamps on at dusk in response to a particular ambient light level, and turn off the lamps at dawn in response to yet another ambient light level. Such controllers are typically photoelectric controllers which are mounted on the lamp housing.
Typical photoelectric controllers now in use exhibit failure rates approaching ten percent. Since photoelectric controllers are generally designed to fail with the load energized or "on", this high failure rate translates into a tremendous amount of wasted electricity. In addition, lamp owners report average labor costs for replacing defective controllers of approximately $50 per unit. Such high replacement cost for a $3 or $4 dollar controller can no longer be tolerated.
Present-day photoelectric controllers exhibit such high failure rates for a number of reasons. For example, a.c. electromechanical relays exhibit a characteristic known as "chattering" as the relay's armature mechanism opens and closes repeatedly as a result of the slowly changing magnitude of the a.c. waveform. This occurs where the electromechanical relay is connected directly to a photocell and control current to the relay changes slowly as the ambient light level changes. This provides no clear, distinct on and off trip point for the contacts, and therefore the contacts arc and ultimately self-destruct.
Thermal bimetallic type controllers have also been used. Although they provide a distinct switching point, the photocell controls current through a heater. This is a slow-reacting switch which may take up to one minute to turn on a lamp or other load. Because of the heat developed in these switches, these controllers also exhibit a very high failure rate. In addition, many photocells exhibit drifting of trigger level due to excessive power dissipation in the photocell or effects from ultraviolet light.
Semiconductor elements such as SCRs have been used in an attempt to replace electromechanical relays. However, since over 100 amps of instantaneous inrush current is often developed through the switch, such a high current level tends to break down and destroy the SCR.